THIS invention relates to a process for recovering valuable metals from ore.
Water and Tailings
Many mineral resources around the world are located in arid terrain where storage of wet tailings consumes excessive water. As an example, around 40% of global copper production is from the Andean desert region of Chile and Peru. As the copper mining industry has developed, the competition for water between mining, agriculture and urban activities has intensified, making permitting of new mining projects problematic. For existing operations, the lack of readily available water is being overcome by utilising ground water (a finite resource). The alternative is desalination of sea water and pumping to the mine site (often located far from the coast or at altitudes in excess of 3000 m). The desalinated water may be sustainable source, but it is a very expensive. Hence access to the mineral endowment in the area is constrained by water.
Similarly, many of the world's gold and copper deposits are in areas where the local terrain and seismic activity makes perpetual storage of large quantities of tailing very problematic.
Given the mountainous terrain, the impoundment of fine tailings for many mines is also difficult. Tailings dams are often located in steep valleys requiring very high dam walls, in areas which have the potential for major earthquakes. Thus, the risk of dam failure, and substantive environmental damage associated with large volumes of fine slurry flowing many kilometers downstream, is ever present. This significant risk is mitigated through highly engineered and regulated tailings impoundment facilities. As such, storage of tailings is often the most expensive part of the overall capital for a new mine.
Using traditional processing and tailings disposal, the tailings storage facility (TSF) also represents the primary sink (up to 80%) for water consumed within the mining process. The hydrophilic nature of fine tailings makes solid/liquid separation expensive through mechanical or chemical means and the fine tailings can contain 0.6-0.7 tonne water per tonne of tailings. The high water content makes the stored material subject to liquefaction in the event of any breach of the dam. Thus, any technique which can minimise the quantity of fine tailings generated will have a major impact on the capital cost of a mining a copper, gold, or mixed copper gold, mineral resource, and a direct effect on the quantity of water required.
With this in mind, some operations cyclone their tailings, to separate around 10-60% of the material as a sand fraction, typically of diameter greater than 100 micron. The sand fraction drains more easily at progressively larger particle sizes, such that water can be partially recovered for recycling by either filtration, screening, or natural drainage from stacking. Typically the remaining fine tailings will have a water content of 65% by weight, whereas on drainage, the fine ±100 micron sand will retain 20-30% by weight water. The sand fraction can be hydraulically stacked; or filtered or screened, and dry stacked. In some cases the sand can be used as part of the TSF dam wall, or else it is stacked separately. The sand fraction not only has a lower moisture content, its larger particle size make it more resistant to liquefaction in the event of an earthquake.
There are also a few small operations which filter all of their conventional flotation tailings for dry stacking, due to specific constraints on tailings storage associated with their location. However, these operations are rare, due to the high cost of filtration of fine tailings material.
Flotation
Flotation has traditionally been used to separate a variety of valuable minerals containing metals such as copper, gold, nickel, platinum group metals, lead, zinc, phosphates, and iron; from the gangue fraction of the ore. The flotation technology creates the conditions for the attachment of an air bubble to a fraction of a finely ground feed, to float one fraction or the other and separate a high grade concentrate from the relatively barren tailings. For example, porphyry ores are typically ground to a diameter of around 50-250 microns to almost fully liberate the copper sulphide mineral particles, and then floated to recover around 90% of the copper as concentrate containing around 25-35% copper.
The processing (crushing, fine grinding and flotation) of such ores has both a high capital cost and high energy consumption. This high cost (around 40% of total cost of a mining and processing operation); dictates in part the cut-off grade of ores which are economic to mine. For this reason, companies have investigated other techniques for physical separation of ore into high grade and low grade streams, prior to grinding to fully liberate the valuable minerals. These physical separation techniques fall under the generic title of pre-concentration, and variously include selective mining, size separation, density separation, or mechanical sorting. Where successful, this upgrading allows either increased overall production through the processing assets, or reduction in the unit cost of processing by reducing the energy required to liberate the valuable mineral. Where pre-concentration is undertaken at a coarse size, the effect is to reduce the material that is ground to a fine size, and hence also reduce the volume requiring special storage as tailings. However, the low selectivity of such pre-concentration techniques usually results in a relatively low recovery of the total resource mined.
Whilst flotation has been used for many years to separate fully liberated ores, coarse flotation of partially liberated ore has not been considered as a viable technology until recently. This is partly due to the difficulty in floating coarse particles, given their tendency to detach from the flotation bubbles, particularly in a highly agitated flotation cell, or through the froth layer designed to improve grades. There is also a trade-off between recovery and grade; i.e. where the valuable particles are only partially liberated from the gangue, flotation does not directly yield both a high recovery and a saleable grade. Regrinding of the material is required to generate a satisfactory concentrate grade.
Recently, some proponents of coarse flotation have been examining opportunities to float at a coarser size fraction for a variety of minerals (Improving the recovery of low grade coarse composite particles in porphyry copper ores Saeed Farrokhpay, Igor Ametov, Stephen Grano Advanced Powder technology 22 (2011) 464-470; Coarse gold recovery using flotation in a fluidized bed; Julio Jairo Carmona Franco, Maria Fernanda Castillo, Jose Concha, Lance Christodoulou & Eric Wasmund, 47th Annual Canadian Mineral Processors Operators Conference, Ottawa, Ontario, Jan. 20-22, 2015; Jameson, G. J., 2010, “New directions in flotation machine design”, Minerals Engineering, Volume 23, pp 835 841; Flotation technology for coarse and fine particle recovery; Eric Bain Wasmund I Congreso internacional de flotacion de minerals, Lima, Peru, August 2014; Flotacion de finos y gruesos aplicada a la recuperacion de minerals de cobre; J. Concha, E. Wasmund). The contents of these documents are incorporated herein by reference. The concept produces an initially low grade concentrate by floating most of the composite particles, and then to mill this low grade concentrate to allow it to be re-floated to form a readily saleable concentrate. The benefits of coarse flotation claimed by the proponents is a reduction in total energy consumed in milling. The residues from both the low grade and saleable flotation circuits as proposed, are sent to a common tailings storage. Thus, the consumption of water and amount of tailings slurry to be stored after this coarse flotation remains the same as for conventional flotation, albeit that the particle size distribution in the tailings would be somewhat coarser.
Coarse flotation has typically targeted a grind to a particle diameter of above 150 microns. The aim is to minimise total cost by reducing grinding energy, and hence the balance between pre-grinding to get a high overall recovery; and the limited mass pull to deliver the reduced energy consumption in fine grinding.
Specific flotation machines have been designed to improve this recovery of coarse mineral particles, including those particles which are not fully liberated from the gangue. These coarse flotation machines typically operate with air sparging in a fluidised bed arrangement, and have a thin or no froth layer to minimise the detachment of target mineral particles as they reach the product layer. Tailings produced from such a coarse and subsequent fine grinding system are a mix of the barren material from coarse flotation, and the barren material from the regrind and re-float.
Despite commercial designs being available for such specific flotation machines, the commercial application has been limited, presumably because the gains in energy efficiency are offset by other factors such as a slight loss in overall recovery. Importantly, in the configurations currently proposed, there are no significant gains achieved in water consumption or tailings storage requirements.
It is an object of the present invention to provide an improved process for recovering valuable metals that results in reduced water consumption and tailings storage requirements.